Researchers have found that women who work night shifts for more than 30 years may be at a higher risk of breast cancer than other women.
The study, published in the British Medical Journal (BMJ), was done by researchers in Canada. They analyzed the careers of 1,134 women with breast cancer and 1,179 women without the disease.
The women, all of the same age, were questioned about their shift work patterns throughout their employment history, and hospital records were used to determine their tumor type.
Around 1 in 3 women in both groups of the study had worked night shifts. The research revealed that women who had worked night shifts for up to 14 years or for between 15 and 29 years had no increased risk of developing breast cancer. But the women who had worked night shifts for 30 years or more were twice as likely to develop the disease.
In the study, the researchers suggest that the night-time artificial lighting that shift workers are exposed to could be one factor giving rise to the cancer link.
The researchers believe that long-term exposure to artificial lighting can suppress the production of melatonin.
Melatonin is a natural hormone made by the body's pineal gland, found in the brain. The pineal gland is inactive during the day. During the night, the gland actively produces melatonin, rising the level of the hormone in the blood and causing a person to feel less alert, encouraging the need for sleep. See the National Sleep Foundation (NSF) for more information on melatonin
Melatonin levels are elevated for around 12 hours throughout the night and reduce to an almost undetectable level during the day. The NSF says it may be that exposure to artificial light during the night inhibits the pineal gland's production of melatonin.
According to the researchers in the BMJ study, lack of melatonin production can lead to increased production of estrogen in the body, and that this may be a trigger for breast cancer in some women. The researchers also discuss whether sleep disturbances, upset body rhythms, lack of vitamin D and lifestyle differences also contribute to the increased risk of breast cancer.
Previous studies have drawn a link between breast cancer and night-shift working in nurses. A 2001 study published in the Journal of the National Cancer Institute showed a similar association to the Canadian study's finding, showing that light exposure during the night can suppress melatonin production in night nurses, perhaps increasing their risk of cancer.
However, this latest Canadian study has analyzed women of various occupations involved in night shifts, meaning the results may be more relevant to the general population.
The study authors say further work is needed to gain a better understanding of exactly how night shift working may be playing a role in breast cancer.
The authors conclude:
"As shift work is necessary for many occupations, understanding which specific shift patterns increase breast cancer risk, and how night shift work influences the pathway to breast cancer is needed for the development of healthy workplace policy."